Cryo-refrigerators are low-temperature refrigerating machines in which thermodynamic cyclic processes take place (see e.g. U.S. Pat. No. 2,906,101). A single-stage cryo-refrigerator usually includes a compressor, connectlng lines, and a cold head with a displacer and a chamber. During operation, the chamber is connected alternatingly with a high-pressure-and a low-pressure helium source, so that during reciprocation of the displacer, a thermo-dynamic cyclic process (Stirling-process, Gifford/Mc Mahon-process etc.) takes place, whereby the operating gas is carried in a closed cycle. The consequence ls that heat is eliminated in a certain region of the chamber. With a two-stage refrigerator of this kind temperatures below 10 K can be reached.
One essential unit for such a refrigerator is the compressor, in which the helium, having expanded in the refrigerator, is compressed again, i.e. from approx. 7 bar (low pressure) to approx. 22 bar (high pressure). Almost all of the energy expended to accomplish this compression is converted into heat energy. Approximately 25% of this heat is carried by the helium and approximately 75% by the oil of the compressor. Usually either water or air is used to cool the hel[um and the oil.
Known compressors that are air-cooled have the disadvantage that they are relatively large. The ventilator, which is located in the compressor housing, generates air movements and dust whirls. Consequently, currently available air-cooled compressors are not useful in environments in which sensitive electronic supply-and measuring devices are located to perform low-temperature experiments. What is particularly disadvantageous is that these air-cooled compressors cannot be employed in dust-free "cleanrooms". The heat which is released into the room poses another significant problem, since the power of standard compressors for cryo-refrigerators usually is in the range of 3 to 7 KW. The heat generated by such compressors causes a considerable temperature increase in the room. This is undesirable particularly in the summer, and can also interfere with the operation of electronic devices.